Japanese Patent Laid-Open No. 287617/1991 proposes as a block polymer a thermoplastic elastomer having a structure represented by the general formula (A-B)n-X (wherein A represents a vinylaromatic polymer block and B represents a diene compound polymer block). However, thermoplastic elastomers of this kind have two or more independent vinylaromatic polymer blocks in the molecule and necessitate an exceedingly high processing temperature due to the cohesive force of the vinylaromatic polymer blocks. It is therefore difficult to compound such elastomer with silica, carbon black, or the like and vulcanize the composition with sulfur.
Recently, many techniques have been proposed for incorporating silica into a raw rubber to produce a rubber composition for tires designed for attaining resource saving and energy saving.
Namely, in those techniques, silica or a combination of silica and carbon black is incorporated into a rubber, in place of the incorporation of carbon black in preparing tire tread rubbers heretofore in use, to thereby obtain a higher impact resilience, heighten the low-temperature (0° C.) tan δ of viscoelasticity for improving wet skid resistance, and simultaneously lower the high-temperature (50–70° C.) tan δ for reducing tire rolling resistance so as to attain a reduced energy loss.
A typical technique is proposed, for example, in U.S. Pat. No. 5,227,425. In this technique, silica is used as a reinforcing filler for an SBR having a specific structure and the rubber composition is kneaded under specific conditions to thereby obtain a tread rubber composition having an improved balance between fuel-saving performance and wet skid resistance.
For improving the dispersibility of silica in such a rubber composition is used a silane coupling agent such as bis(triethoxypropyl) tetrasulfide.
Furthermore, techniques for the terminal modification of a rubber with various alkoxysilyl groups and silica-containing rubber compositions produced with these techniques are proposed in Japanese Patent Laid-Open Nos. 227908/1987, 53513/1996, 53576/1996, 225324/1997, etc. These techniques are intended to improve the dispersibility of silica in rubbers and to reduce the amount of the silane coupling agent to be used.
These alkoxysilyl-modified polymers are obtained by reacting a terminal-active polymer obtained by anionic polymerization with a specific alkoxysilane compound.
The rubber compositions according to the various techniques described above are intended mainly to improve the balance between the wet skid resistance and rolling resistance of tires. Namely, those techniques are intended to modify viscoelastic properties so as to heighten the low-temperature (0° C.) tan δ for improving wet skid resistance and lower the high-temperature (50–70° C.) tan δ for reducing rolling resistance.
However, such rubber compositions have problems, for example, that performances change considerably according to conditions, e.g., with changing ambient temperature. For example, the rubber in a low-temperature range (e.g., 0° C.) has an increased hardness and is reduced in rubber elasticity, e.g., impact resilience, due to the heightened tan δ in the low-temperature range. In a high-temperature range (50–70° C.), on the other hand, the rubber has a lowered tan δ and, hence, has a reduced hardness.